The present invention relates generally to improved jackets for insulated electric cables, and more particularly to jacket materials containing additives that effectively remove or inhibit the passage of ionic impurities into the insulation of the cable which can weaken the insulative properties.
Electric cables have been used extensively since about 1960 and were originally intended to be serviceable for a period in excess of 40 years. However, there have been premature failures of these cables, the major cause of which has been attributed to the formation of water trees, which are dendritic structures within the insulative layers that considerably weaken the dielectric properties of the cable insulating materials. The points of initiation of the water trees seem to be defects in the insulation such as impurities, aggregated admixtures, voids, gaps, cracks or boundary surfaces. While the mechanism of water tree formation is not fully understood, some studies have demonstrated that ionic impurities contained in groundwater are major promoters of water treeing. Insulated cables typically have an outer jacket which provides protection against physical damage and water diffusion into the insulation when the cable is buried. However, the many commonly used cable jacket materials still apparently do not adequately protect the insulation materials from water treeing.
There have been several attempts to solve this problem. In some cases a thin metal water barrier, typically made of lead or aluminum is placed between the jacket and the insulation shield. While such metal barriers prevent the ingress of water, they add considerably to the manufacturing cost of the cable. Moreover the use of metal barriers generates a set of technical problems, including metal corrosion and/or cracking due to thermal expansion and contraction. The metal barriers also add significant weight to the cable. For these reasons, use of a metal water barrier beneath the jacket has not been widely accepted.
A typical cable comprises a conductor core, surrounded by a conductor shield (usually a thin layer of semiconducting material which is compatible with the conductor), followed by insulating material, a second shield (which is another layer of semiconducting material used to cover and protect the cable insulation), a set of helically applied copper conductors or tapes (used as a ground or neutral conductor), then a jacket, usually extruded over the copper conductors, to impede the ingress of water. As another proposed solution for protecting cables, the conductor strands may be treated with water blocking compounds such as polymeric sealants. However, the long term effectiveness of these compounds for use for that purpose is not known.
The present invention provides an improved jacket which prevents or significantly retards the ingress of harmful ionic impurities from passing through the jacket into the insulation materials, thereby preventing or impeding the growth of water trees.